1. Field of the Invention
The invention relates to a measuring device for electrical power and to a method for measuring the electrical power. The measuring device has a multiplexer connected to an analog-to-digital (A/D) converter. The multiplexer receives current and voltage signals that are digitized by the A/D converter. A digital processing device controlled by a control unit further processes the digitized signals for formulating power values.
In order to determine or measure electrical power of an electrical load, it is known to feed the current and voltage signals acquired at the load in each case to an analog to digital converter (A/D converter) and to multiply them by one another in a downstream digital processing device.
Contemporaneous values of current and voltage are combined with one another in the process.
In order to compensate for angle errors originating, by way of example, from a shunt or a current transformer, a digital filter may be connected downstream of the A/D converter. As an alternative, external compensation using RC circuitry (analog filter) is also possible. A measuring device of this type requires a high outlay on components, so it is appropriate essentially only for high-quality meters having a high level of accuracy.
German Patent DE 26 30 359 C2 discloses a measuring device in which an analog time delay element is used to compensate for a time delay between two signals, which time delay element is connected upstream of a multiplexer.
Published, European Patent Applications EP 0 681 235 A1 and EP 0 634 662 A1 each disclose an electricity meter in which an A/D converter, as in the case of Published, European Patent Application EP 0 377 282 A1, corresponding to U.S. Pat. No. 5,017,860, is used in each case for current and voltage measurement. The A/D converter is in each case configured as a sigma-delta converter.
Published, Non-Prosecuted German Patent Applications DE 195 21 609 A1 and DE 195 21 610 A1 disclose, in connection with electrical energy measurement, using a decimation filter which has a selectable decimation ratio. A reference to the problems in the present case is not provided therein.